This invention relates generally to acetals of sorbitol and xylitol useful as clarifying agents for crystalline polyolefin resins, and particularly, to the use of clarifying agents which have been reduced to their primary particle size--ultrafine powders.
The use of clarifying agents to reduce the haze in articles manufactured from crystalline polyolefin resins is well known in the art. Representative acetals of sorbitol and xylitol, which have been employed as clarifying agents, include the following U.S. patents:
Hamada, et al., U.S. Pat. No. 4,016,118; Dibenzylidene sorbitols PA1 Kawai, et al., U.S. Pat. No. 4,314,039; Di(alkylbenzylidene) sorbitols PA1 Mahaffey, Jr., U.S. Pat. No. 4,371,645; Di-acetals of sorbitol having at least one chlorine or bromine substituent PA1 Kobayashi, et al., U.S. Pat. No. 4,532,280; Di(methyl or ethyl substituted benzylidene) sorbitols PA1 Williams, et al., U.S. Pat. No. 4,845,137; Dibenzylidene sorbitol derivatives having at least one substituent group containing sulphur PA1 Kobayashi, et al., U.S. Pat. No. 4,954,291; Distribution of diacetals of sorbitol and xylitol made from a mixture of dimethyl or trimethyl substituted benzaldehyde and unsubstituted benzaldehyde PA1 Rekers, U.S. Pat. No. 5,049,605; Bis(3,4-dialkylbenzylidene) sorbitols including substituents forming a carbocyclic ring
Additionally, generic structures for clarifying agents useful in polypropylene are disclosed in published Japanese applications Mitsubishi Petroch KK, No. 85-157213/26; and Sumitomo Chem Ind. KK, No. 88-130662/19. Techniques for manufacturing the clarifying agents are disclosed in the above references, and in Murai, et al., U.S. Pat. No. 3,721,682; and New Japan Chemical Company, U.K. Patent Application, GB 2,115,405 A. All of the above patents and published applications are incorporated by reference herein.
Although the exact mechanism is not well understood, it is generally believed that the clarifier must melt and recrystallize to form a very fine network within the polyolefin resin. This crystalline network provides nucleation sites, which reduces the size of the spherulites formed in the resin as it cools. Small spherulites do not scatter visible light as effectively as large spherulites, so the nucleated polyolefin resin has improved clarity.
Clarified polypropylene is produced by blending a clarifying agent with the base polyolefin resin, along with other additives such as antioxidants, acid scavengers and lubricants, and then extruding the mixture at a temperature above the melting point of the clarifying agent. A more popular method for producing clarified polyolefin resin involves pre-blending all or some of the additives with a portion of the base resin to make a powder master batch. The master batch is metered into the extruder with additional base resin to eliminate the need for large mixers. The extrudate is usually formed into small pellets. Alternatively, the master batch itself may be extruded and pelletized. These pellet concentrates may be mixed with polyolefin resin which has been extruded without additives to obtain a product having the desired concentration of clarifying agent, generally from 0.01 up to 2 or 3 wt. %.
There are a number of difficulties associated with the use of sorbitol and xylitol acetal clarifying agents in polyolefin resin. One major problem is the formation of "white points" or bubbles in articles fabricated from these resins. Small bubbles in the side walls of injection molded housewares and medical devices are considered a major defect, so several methods have been employed to minimize this problem.
One approach found through experimentation is to add small quantities of polar fatty additives, such as glycerol monostearate or fatty amides, to the sorbitol acetal clarified polyolefin resin. These additives reduce the number of bubbles observed in fabricated parts but do not eliminate the problem. Also, polar fatty additives tend to "bloom" or migrate slowly to the surface of the fabricated parts and form a waxy build-up, which is undesirable.
A second approach used to solve the bubble problem with sorbitol and xylitol acetal clarified polyolefins is to melt compound the resin 3.degree. C. to 10.degree. C. above the melting point of the clarifying agent. While this solution has been workable, it has several major drawbacks. Sorbitol acetal clarifiers typically have melting points 50.degree. to 100.degree. C. higher than the polyolefin resins with which they are compounded. Compounding the polyolefin resin above the melting point of the clarifier can cause color and odor formation in the plastic. Also, it is quite difficult to control the temperature in a large production extruder, so there usually is some off quality produced during the start of a campaign. Sorbitol and xylitol acetal clarifiers tend to boil or sublime near their melting point. Compounding above the melting point of the clarifier can cause plate out at the extruder die, which is undesirable.
A third approach used to eliminate bubbles or "fish eyes" with sorbitol acetal clarified polyolefins has been described by Kobayashi, et al., U.S. Pat. No. 4,954,291 (especially columns 1,2,3,4). This method involves using a distribution of di-acetals of sorbitol made from a mixture of benzaldehyde and di- or tri-methyl substituted benzaldehyde. The composition has a relatively low melting point, but still must be compounded above its melting point to avoid bubbles. Also, the composition has relatively poor clarifying properties compared to di-acetals of sorbitol made entirely from alkyl substituted benzaldehydes.
The problem of "white points" or bubbles in clarified polyolefin resins has received a great deal of investigation. Nevertheless, the exact mechanism of bubble formation and the role that polar fatty additives play in helping to eliminate them are not well understood.
In addition to di-acetals of sorbitol and xylitol, salts of aromatic carboxylic acids, such as sodium benzoate, have been employed as nucleating agents in polyolefin resin with some success. Unlike the acetals, which are compounded at temperatures above their melting point and recrystallize in the resin to provide nucleation sites, sodium benzoate, with a melting point greater than 300.degree. C., does not melt during compounding and typically will decompose before melting. Further, sodium benzoate has been found to be insoluble and immiscible in polyolefins. Therefore, the performance of sodium benzoate as a nucleating agent is dependant upon its dispersion in the polymer melt in as fine a form as possible; in the range of 1 to 10 microns. "Plastic Additives Handbook", Gachter et al. editor, Hanser Publishers, Munich, Germany, pp. 671-683 (1985); and Binsbergen, "Heterogeneous Nucleation in the Crystallization of Polyolefins (1)", Polymer 11, pp. 253-267 (1970). Conversely, the nucleation effects of di-acetals of sorbitol and xylitol appear to be largely independent of their physical characteristics prior to compounding, given the requirement that they are dispersed and recrystallized in the polyolefin resin.
The present invention provides a technique to process sorbitol and xylitol acetal clarifiers so they can be compounded with polyolefin resins to produce fabricated parts without "white points" or bubbles without the use of excessive compounding temperatures which can cause discoloration and odor.